With the recent increase in information processing capacity, various types of information recording technologies have been developed. In particular, the surface recording density of an HDD using a magnetic recording technology has been continuously increasing at an annual rate of about 100%. Recently, 2.5-inch-diameter magnetic disks used in HDDs and the like are required to have an information recording capacity higher than 160 GB per disk. In order to meet such a demand, it is necessary to achieve an information recoding density higher than 250 GBits per square inch.
In order to achieve a high recording density in a magnetic disk used in an HDD or the like, a magnetic disk using a perpendicular magnetic recording system (perpendicular magnetic recording disk) is recently proposed. In a conventional in-plane magnetic recording system, an axis of easy magnetization in a magnetic recording layer is oriented along a plane of a substrate surface. In the perpendicular magnetic recording system, it is arranged that the axis of easy magnetization is oriented to be perpendicular to the substrate surface. As compared with the in-plane magnetic recording system, the perpendicular magnetic recording system is capable of more effectively suppressing a thermal fluctuation phenomenon during high-density recording. Therefore, the perpendicular magnetic recording system is suitable for an increase in recording density.
Heretofore, CoCrPt—SiO2 or CoCrPa—TiO2 is widely used as the magnetic recording layer. In such layer, Co crystals of an hcp (hexagonal closest packed crystal lattice) structure grow in a columnar shape and Cr and SiO2 (or TiO2) segregate to form a nonmagnetic grain boundary. By the use of such a granular structure, fine magnetic grains physically independent from one another are easily formed so that a high recording density is easily achieved.
In the above-mentioned perpendicular magnetic recording system, a single pole perpendicular head is used to generate a magnetic field perpendicular to the magnetic recording layer. However, if the single pole perpendicular head is merely used, magnetic flux from a single pole end directly returns to a return pole on an opposite side so that a magnetic field of a sufficient intensity can not be applied to the magnetic recording layer. In view of the above, the perpendicular magnetic recording disk is provided with a soft magnetic layer under the magnetic recording layer and a path of the magnetic flux (magnetic path) is formed in the soft magnetic layer. With this structure, it is possible to apply an intense perpendicular magnetic field to the magnetic recording layer. Specifically, the soft magnetic layer is a layer in which a magnetization direction is aligned in response to a magnetic field during writing and a magnetic path is dynamically formed.
However, an intense magnetic field applied to the magnetic recording layer results in an increase in leakage magnetic field to an adjacent track. This results in a problem of WATE (Wide Area Track Erasure), i.e., a phenomenon of erasure of recorded information over an area within several micrometers from a track to be subjected to writing. As a technique for reducing WATE, it is important that a reverse magnetic domain nucleation field Hn of the magnetic recording layer has a negative value and its absolute value is large. In order to obtain high Hn (having a large absolute value), proposal is made of a CGC (Coupled Granular Continuous) medium comprising a magnetic recording layer having a granular structure and a thin film (continuous layer) which is formed above or below the magnetic recording layer and which exhibits high perpendicular magnetic anisotropy (Patent Document 1).
If a coercive force Hc of the magnetic recording layer is increased, an increase in recording density can be achieved but writing by the magnetic head tends to become difficult. In view of the above, the continuous layer also serves to improve writabiliy, i.e., overwrite characteristic by improving saturation magnetization Ms.
In other words, the purpose of forming the continuous layer on the magnetic recording layer is to reduce noise by improving the reverse magnetic domain nucleating field Hn and to improve the overwrite characteristic by improving the saturation magnetization Ms. The continuous layer may be called an auxiliary magnetic layer or a cap layer. In this application, the term “continuous layer” is used unless specifically indicated otherwise.
Patent Document 1: JP-A-2003-346315